The present invention relates to thin-film amorphous silicon solar cells and to a method for forming a silicon-oxide semiconductor film suitable for use as a window layer in an amorphous silicon (a-Si) solar cell.
Large area films of amorphous silicon (a-Si) as the main material can be readily made via a glow discharge decomposition of a silane-containing raw material gas or a photo-CVD process. The combination of large area and easy processing makes amorphous silicon a desirable candidate for use in low-cost solar cells. Such a solar cell generally employs a p-i-n junction construction as shown in FIG. 6. The solar cell of FIG. 6 is manufactured by laminating sequentially, on a glass substrate 1 a transparent electrode 2, a p-type a-Si layer 31, a p/i interface layer 4, an intrinsic a-Si layer 5, an n-type a-Si layer 6, and a metallic electrode 7. This solar cell generates electric power by light admitted through the glass substrate 1.
In this construction, photo-carriers that contribute to power generation are generated mainly at the intrinsic layer, while the p-type layer and the n-type layer are dead layers. Therefore, in a solar cell such as that of FIG. 6 in which light passes through the p-type layer to reach the intrinsic layer, it is important in order to increase the output of the device to raise the light transmissivity in the p-type layer so that as much light as possible can reach the intrinsic layer. To achieve this, it is effective to increase the optical gap, Eg, in the p-type layer and thereby decrease optical absorption loss. This can be achieved by adding carbon atoms to the p-type a-Si layer (Japanese laid-open application No. 56-64476), adding nitrogen atoms to the p-type a-Si layer (Japanese laid-open application No. 57-181176), adding oxygen atoms to the p-type a-Si layer (Japanese laid-open application No. 56-142680), or adding oxygen atoms and carbon atoms to the p-type a-Si layer (Japanese laid-open applications Nos. 58-196064, and 61-242085).
Increasing the optical gap of the p-type layer by adding a large quantity of C or N during the formation of the p-type layer does not produce a preferred device, however, because the process results in high levels of dangling bonds in the film, which in turn results in reduced electric conductivity. An a-Si solar cell is usually limited in that the electric conductivity of the film used in the p-type layer at 25.degree. C. must be higher than 10.sup.-8 S/cm to suppress the reduction in the fill factor. When C or N are added to a p-type layer, this requirement can only be met when the optical gap is kept below 2 eV. Therefore, the resulting cell has a relatively large light absorption loss in the p-type layer at 1-2 mA/cm.sup.2 when converted to a short circuit current density. Similar difficulties can be anticipated for the cells disclosed in Japanese laid-open application 61-242085, where an undoped a-Si:O:C:H film is reported to have electric conductivity (.delta..sub.d) of about 10.sup.-12 to 10.sup.-13 S/cm in a region with an optical gap of 2.0 eV or higher when an AM1 (pseudo-solar light) is irradiated at 100 mW/cm.sup.2, although the optical gap of the p-type layer when it is doped is not reported.
A recent report indicates that a micro-crystalline phase has successfully been included in an amorphous silicon carbide (a-SiC) film to which carbon atoms were added, by using the ECR-CVD process shown in Japanese laid-open application 64-51618 or the plasma CVD process described in the Technical Digest of the International PVSEC-3 (1987), p. 49. The inclusion of the Si micro-crystalline phase enables the production of film having increased light conductivity and better electrical characteristics.
Utilization of the Si micro-crystalline phase enables one to widen the optical gap without decreasing light conductivity. Therefore, this has been thought to be a promising method for improving the characteristics of the window layer of a solar cell. The commercial applications of this presently known method are limited, however, because the film forming conditions for micro-crystallization are narrow in scope, thus making it difficult to raise the deposition rate.
Based on the above, the present invention is intended to provide a thin-film solar cell with a higher efficiency using an a-Si-based film that has larger optical gaps as a window layer and a high electric conductivity preferably of 0.5 to 1.times.10.sup.-6 S/cm.
Additionally, the present invention also provides a method for forming an a-Si based film which utilizes an amorphous silicon oxide including a silicon micro-crystallization phase, thereby enabling the production of a film with the desired characteristics, which can be produced over a broader range of industrial conditions than provided for by the above-referenced prior art.